Fix #123769: Assert when saving a file with deleted workspaces.

Delete sceens when their workspace is deleted.

Also remove `IDTYPE_FLAGS_NEVER_UNUSED` tag from bScreen ID type, there
is no more point to keep these IDs around if they are not used, there
are supposed to be mere sub-data of the Workspaces.

Pull Request: https://projects.blender.org/blender/blender/pulls/129975

source/blender/blenkernel/intern/screen.cc

@@ -170,7 +170,7 @@ IDTypeInfo IDType_ID_SCR = {
     /*name_plural*/ N_("screens"),
     /*translation_context*/ BLT_I18NCONTEXT_ID_SCREEN,
     /*flags*/ IDTYPE_FLAGS_NO_COPY | IDTYPE_FLAGS_ONLY_APPEND | IDTYPE_FLAGS_NO_ANIMDATA |
-        IDTYPE_FLAGS_NO_MEMFILE_UNDO | IDTYPE_FLAGS_NEVER_UNUSED,
+        IDTYPE_FLAGS_NO_MEMFILE_UNDO,
     /*asset_type_info*/ nullptr,
 
     /*init_data*/ nullptr,

source/blender/editors/screen/workspace_edit.cc

@@ -257,6 +257,12 @@ bool ED_workspace_delete(WorkSpace *workspace, Main *bmain, bContext *C, wmWindo
     }
   }
 
+  /* Also delete managed sceens if they have no other users. */
+  LISTBASE_FOREACH (WorkSpaceLayout *, layout, &workspace->layouts) {
+    BKE_id_free_us(bmain, layout->screen);
+    layout->screen = nullptr;
+  }
+
   BKE_id_free(bmain, &workspace->id);
   return true;
 }